Flight paths are generally calculated in three dimensions, i.e. altitude and lateral position. To calculate a flight path in four dimensions requires the three-dimensional position of the aircraft to be specified over a number of points in time.
The ability to fly an aircraft according to a predetermined flight path with accuracy such that its position as a function of time is predictable is becoming increasingly important in air traffic control. This would allow air traffic control to relax separations between aircraft, leading to more efficient use of air space.
Typically, aircraft approach an airport under the guidance of air traffic controllers. The air traffic controllers are tasked with ensuring the safe arrival of aircraft at their destination, while also ensuring the capacity of the airport is maximised. The former requirement is generally met by ensuring minimum specified separations are maintained between aircraft. Air traffic control is subject to uncertainties that may act to erode the separation between aircraft such as variable winds, both in speed and direction, and different piloting practices. Nonetheless, large numbers of aircraft can operate safely confined in a relatively small space since air traffic control can correct for these uncertainties at a tactical level using radar vectoring, velocity change and/or altitude change. A typical approach to an airport will involve a stepped approach where the aircraft is cleared to descend in steps to successively lower altitudes as other air traffic allows.
Most commercial aircraft are equipped with flight management systems which offer automated navigation in descent. For example, FIG. 1 shows how such a system might operate. At step 101 the flight management system plans a reference trajectory for the descent into an airport. The reference trajectory will be in accordance with air traffic management requirements and rules set by the flight management system. The aircraft is automatically directed along the trajectory. The system monitors for real-time deviations from the reference trajectory at step 102. The flight management guidance system corrects for deviations at step 103, by adjusting one or more flight parameters at the expense of others. For example, position may be maintained at the expense of timing, or vice versa.
One particular example of a flight management system is an automated vertical navigation (VNAV) system with different modes, such as VNAV PATH which is typically used in descent. VNAV PATH uses a path-on-elevator method to track a vertical reference profile. In this mode the aircraft tracks a reference potential energy profile, while the engines remain at a reference idle power. Unexpected energy deviations, such as caused by errors in the predicted wind strength and/or direction, will affect the kinetic energy. That is, the ground speed of the aircraft maybe changed because of the wind strength, which in turn results in changes to the aircraft position over time.
A different mode of operation of a flight management system is VNAV SPEED which is controlled by comparing how the airspeed at the elevator tracks an airspeed reference profile. In this mode unexpected energy deviations primarily result in potential energy deviations along with some kinetic energy deviation arising because of the relationship between airspeed and ground speed.
A more advanced guidance method used by Boeing 737s is a Required Time of Arrival (RTA) method which combines VNAV PATH with path recalculations to meet a target time of arrival at a given waypoint.
Furthermore, under current research is a four-dimensional guidance method in which the aircraft tracks a ground speed reference. In the method all errors are pushed into the vertical position of the aircraft using the elevators.
Other four dimensional guidance methods have been developed which apply energy corrections to the aircraft using spoilers and throttles respectively to decrease and increase kinetic energy. Further four dimensional guidance methods attempt to meet a target arrival time which apply energy corrections by deploying high-lifting devices (flaps) and landing gear earlier or later in the descent.
Ultimately the various different guidance methods, principals and systems each have some combination of vertical and temporal deviation from a reference trajectory. Statistics on these vertical and temporal deviations will result in predicted vertical and temporal actual navigation performance, known as V-ANP and T-ANP. By also considering external factors which have various inaccuracies, for example in predicted wind speed, or aircraft mass, further deviations will arise.
In future Super-Density Operations as part of the next generation of air traffic management it is expected that four-dimensional trajectories will be contracted between the air and ground. Limitations to any deviations to the trajectories must be set to avoid conflicts in the airspace, especially when aircraft are merging, are in-trail, or when crossing traffic. An aim of the next generation of air traffic management standards is also to be able to accommodate an increased throughput of aircraft to cope with a growth in traffic. This might be achieved by setting more stringent specifications on aircraft arrival time. Hence, the air navigation service provider (ANSP) will need to set a specification for the required temporal and vertical navigation performance (T-RNP and V-RNP) similar to current specifications set for lateral required navigation performance. In the US the Federal Aviation Authority has requested information from the aircraft and air traffic management industry so that it can ascertain and ultimately set reasonable navigation requirements without excessively constraining airport operations. Hence, there is a need for a technique for considering and assessing suitable vertical and temporal navigation performance (V-RNP and T-RNP) requirements.
The definition of appropriate values for vertical and temporal navigation performance (V-RNP and T-RNP) is not straight forward because they cannot be set independently of each other. As described above, energy conservation principles provide links between them. That is, the time of arrival at a particular waypoint can be brought forward by speeding up the aircraft, for example by pointing the nose down to lose height and increase the aircraft velocity. Furthermore, certain combinations may not be viable and will be dependent on the type and equipment levels of the aircraft.
Another factor to be taken into account is the operational requirements of a given airport. For example, a low traffic airport in a city or metro environment may need to set more stringent V-RNP at the expense of relaxed T-RNP, whereas a heavy traffic airport may set a tight T-RNP during peak time hours. In particular, the T-RNP may limit the aircraft types that can meet the navigation requirements during that time. But such a T-RNP would need to be relaxed at other times because otherwise the types of aircraft entering that airport would be restricted.
Currently, vertical navigation performance is only evaluated in terms of the accuracy of the flight guidance such as controlled by, for example, a path-on-elevator method. In other words the vertical performance is assessed by how accurately the aircraft tracks the vertical part of the reference trajectory in the presence of flight technical errors and navigation system errors. Research on temporal navigation performance has focussed on delivery/timing accuracy at specific waypoints, such as the runway threshold or meter fixes. The evaluation of both vertical and temporal accuracy of vertical guidance methods with respect to a descent profile between the air and ground adds new complexity.
As mentioned above the V-RNP and T-RNP are linked to each other to such an extent that the specification of limits for one impacts the other. In order to aid the air navigation service providers (ANSPs) in setting limits on V-RNP and T-RNP it would be useful to be able to see their relationship and how the various flight management systems and control modes affect the relationship.